The vapor phase oxidation of ethylene to acetaldehyde is well known. For example, in the Wacker process, as illustrated by U.S. Pat. No. 3,131,223, ethylene together with an oxygen containing gas and water vapor are passed over a noble metal compound catalyst to produce the aldehyde. The use of redox components to maintain the noble metal catalyst constituent in the oxidized state (U.S. Pat. No. 3,131,223; 3,057,915; and 3,301,905), with acids, e.g., sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid or acetic acid, to increase anion concentrations (U.S. Pat. No. 3,131,223 or 3,057,915) has been described as increasing catalytic activity in such operations. The use of a noble metal catalyst is disclosed in U.S. Pat. No. 3,439,044. In all of the above processes, however, the principal reaction product formed is acetaldehyde, at most only small amounts of acetic acid being produced as a by-product in the reaction.
U.S. Pat. No. 3,534,093 discloses a process for the preparation of acetaldehyde, acetic acid and acetic anhydride by the oxidation of ethylene in the presence of a palladium metal or palladium-gold alloy catalyst.
Commercial techniques for the manufacture of acetic acid from ethylene have involved hydrating the olefin to ethanol followed by dehydrogenation and oxidatively dehydrogenating the ethanol to acetaldehyde and then oxidizing the acetaldehyde to acetic acid, or initially oxidizing the ethylene to acetaldehyde followed by a second oxidation under different conditions to obtain the acetic acid.
A one-step vapor phase process for the oxidation of ethylene, in the presence of a catalyst containing palladium chloride and a vanadium or molybdenum oxide, is described in U.S. Pat. No. 3,240,805. The use of a combination catalyst comprising a noble metal compound, a compound of a transition metal of Groups I, VII and VIII, an alkali metal compound and a compound of a transition metal element of Groups III-VI for such purpose is taught in U.S. Pat. No. 3,293,291. A further variation using a catalyst mixture of a salt or coordination compound of palladium and a carboxylate of iron, cobalt or manganese is disclosed in U.S. Pat. No. 3,459,796. British Pat. No. 1,142,897 suggests employing yet another catalyst system, viz, a support mixture of palladium metal and a transition metal oxide or salt, for the ethylene base production of acetic acid. In general, however, commercial adoption of these procedures has not been possible because of the relatively large amounts of by-product acetaldehyde simultaneously produced therein and/or the concomitant large losses of ethylene reactant attributable to combustion to carbon dioxide.
Reyerson and Swearingen, J. Am. Chem. Soc. 50, 2872, observed that ethylene is effectively oxidized to carbon dioxide and water at 100.degree. C. in the presence of supported palladium or platinum catalysts. Our work has confirmed these observations and has also shown that the same products are obtained without regard to the nature of the support.
U.S. patent application Ser. No. 197,528, now U.S. Pat. No. 3,792,087, discloses a vapor phase process for oxidizing ethylene to acetic acid with oxygen in the process of palladium metal supported on a carrier and impregnated with phosphoric acid. Phosphoric acid is apparently unique, since other phosphates, pyrophosphates, organic phosphorus compounds, etc., likely do not function in this oxidation system.
In order to achieve the selective oxidation of ethylene in the presence of a noble metal catalyst, one must first control the complete oxidation reaction, and then selectively initiate the partial oxidation. We have now found that certain sulfur compounds will moderate noble metal catalysts so as to yield selective oxidation catalysts and these catalysts can be used in either the vapor phase, liquid phase or trickle phase reaction systems.
It is accordingly a principal object of the present invention to provide a process for the selective preparation of acetic acid by the oxidation of ethylene with attendant low production of combustibles and acetaldehyde. Further objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the preferred forms thereof.